**10. Conclusions and future perspectives**

between the age of 30 and 49 because he will naturally die before he reaches a 50% chance of becoming sick. However, *a 157 Bq kg−1 diet is not negligible for this average person because there is* 

Additionally, the number of days (or years) required to reach the TD50 value when 100 Bq kg−<sup>1</sup>

*male human consumes a 100 Bq kg−1 diet*, *it takes 16.7 years to reach the TD50 value*. This is a non-

Considering that the amount (becquerel) of radioactivity concentration of 134Cs and 137Cs discussed above is as low as the amount of naturally occurring 40K, a counter argument to this discussion would be that no harmful effect is expected from the conventional dosimetric view. However, it should be remembered that the amount of radiocesium is simply an indication of pollution levels in terms of the field effects. Moreover, we have experimental evidence that artificial radiocesium is clearly harmful at radioactivity levels as low as those observed for radiopotassium (unpublished data). I will discuss this important issue if there is an opportunity to do so in the future.

It should also be remembered that the discussion above completely ignored the dose-rate effects and the physiological differences between butterflies and humans, which include different biological half-lives and organ accumulation of cesium species. This study also ignored the different types of indirect field effects that may be species-specific, depending on the ecological status of a species. It should also be noted that *the TD50 state is toxicologically convenient to evaluate potential effects*, *but it means a devastating massive outbreak of diseases in terms of public health*. Another viewpoint to consider is that toxicological evaluations are often misleading and give the impression that anything that does not reach the TD50 value within a reasonable time or does not exceed the limit is completely safe for everybody. Scientists and politicians should pay special attention to minorities who may still be affected at this level [48, 96].

**Figure 3.** Extrapolation of toxicological data from the pale grass blue butterfly to an average Japanese male human. (a) Linearly extrapolating the butterfly data to understand the relationship between radioactivity concentration in consumed diet and time to reach TD50. For example, to reach the TD50 value in 10 years, an average daily consumption

consumption and 10 Bq kg−<sup>1</sup>

of 100 Bq diet is consumed daily, it takes 16.7 years for a Japanese male human to reach the TD50 value (8.9 × 10<sup>5</sup> Bq

diet is required. (b) Linear relationship between cumulative radioactivity in a body and

consumption are shown. When an average

diet is consumed can be calculated (**Figure 3b**). *When an average Japanese* 

diet may be negligible because it takes 167 years

*still a 50% chance of becoming sick in the next 10 years*.

to reach the TD50 value, which is beyond the human lifespan.

negligible time span. However, a 10 Bq kg−<sup>1</sup>

diet or 10 Bq kg−<sup>1</sup>

62 New Trends in Nuclear Science

of a diet containing 157 Bq kg−<sup>1</sup>

body−<sup>1</sup> ).

time to reach TD50. Lines with daily 100 Bq kg−<sup>1</sup>

It can be concluded that the "low-dose" exposure from the Fukushima nuclear accident imposed potentially non-negligible toxic effects on organisms including butterflies and humans through field effects. At the high-dose exposure, the same field effects would exist, but they would likely be masked by the acute damage. The direct effects may be assessed reasonably by dosimetric analysis even in the field cases, especially for high-dose cases. The field-laboratory paradox is not really a paradox; rather, it indicates our fragmentary knowledge on the real-world pollution caused by this nuclear accident.

Although this chapter sheds light on one important low-dose issue, there are many other issues associated with the field effects that should be studied both in the field and in the laboratory. One of these issues is the *adaptive and evolutionary responses* of organisms to environmental radiation in contaminated areas. The pale grass blue butterfly appears to have evolutionarily adapted to the environmental pollutants [98]. This adaptive evolution may be largely in response to the field effects because the butterfly is essentially very resistant to direct irradiation without any possible adaptive response (unpublished data). However, the direct ionizing damage on DNA would also play an important role in adaptive response if such damage exists.

Simply because there are multiple effective pathways of the field effects, *sensitivity variations* to different modes may vary considerably among species and even among individuals in a given species. The net effects may be determined through synergistic amplification. To further understand the effects of the Fukushima pollution, multifaceted scientific approaches that are firmly based on field work and field-based laboratory experiments (such as the internal exposure experiments using the field-harvested leaves) are expected in the future. A mechanistic understanding of the indirect field effects is also necessary to advance this field of pollution biology.

Simultaneously, studies on the mechanisms of the direct ionizing effects in the field (although the final effects may also be affected by the indirect field effects) should be advanced. As pointed out by Steen [99], multifaceted analyses at the DNA and genomic levels are expected to reveal evidence for direct DNA damage in the field after the Fukushima nuclear accident. I believe that the immediate early exposures to short-lived radionuclides impacted DNA directly, which then might have been inherited to subsequent generations. Such evidence would firmly establish the adverse biological effects caused by the Fukushima nuclear accident at the molecular level. Furthermore, spatiotemporal changes of such DNA damage would reveal population-level dynamics of adaptive evolution in the field, serving as an important case of the real-world evolution in evolutionary biology as well as in radiation pollution biology. Borrowing the famous phrase from *Hamlet* again, I would state, "*To be and not to be* (i.e., the direct and indirect field effects)*, that is the answer*".

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Understanding Low-Dose Exposure and Field Effects to Resolve the Field-Laboratory Paradox…

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